The inboard portion of a wind turbine blade is made thick to support thrust and lift loads that are cantilevered onto the blade root by the outboard blade regions. Herein “inboard” means radially inward toward the blade root, which is the portion of the blade connected to the hub. “Outboard” means radially outward, or toward the blade tip. The inboard portion of each blade becomes progressively thicker perpendicular to the airfoil chord toward the hub for strength, and typically becomes cylindrical adjacent to the hub to facilitate mounting and interface with a blade pitch adjustment mechanism. The relative air inflow angle changes with distance from the rotation center due to the increasing blade speed relative to the incoming wind. For manufacturing reasons, the chord angle or twist angle of the blade cannot change fast enough along its radial length to provide an optimal orientation of the blade airfoil section to the relative air inflow direction, resulting in an increasingly excessive angle of attack proximate the root. These inboard portions experience high variations of angle of attack due to the coning angle, wind speed variations, and low speed of the blade. The thick profiles and non-optimal airfoil shapes necessitated by structural reasons can not produce enough lift over the wide range of angles of attack and can even stall. Due to the blade thickness, structural limitation in airfoil shape, and high angle of attack, the inboard portion of the blade is aerodynamically inefficient and may even be permanently stalled, reducing the wind energy conversion efficiency. Herein “angle of attack” means the angle between the airfoil chord line and the relative wind vector, taking the blade rotation into account. A stalled condition occurs when the angle of attack is too high and the air passing over the suction side of the airfoil detaches from the surface of the blade, creating a separated flow region. Thus, the inboard region of the blades produces low lift and consequently low torque, and it therefore contributes little to the power of the wind turbine. Flow altering devices including slats and flaps have been added to wind turbine blades to improve their local and overall aerodynamic performance.